The importance of clean semiconductor workpiece surfaces in the fabrication of semiconductor microelectronic devices has been recognized for a considerable period of time. Over time, as VLSI and ULSI silicon circuit technology has developed, the cleaning processes have gradually become a particularly critical step in the fabrication process. It has been estimated that over 50% of the yield losses sustained in the fabrication process are a direct result of workpiece contaminants. Trace impurities, such as sodium ions, metals, and particles, are especially detrimental if present on semiconductor surfaces during high-temperature processing because they may spread and diffuse into the semiconductor workpiece and thereby alter the electrical characteristics of the devices formed in the workpiece. Similar requirements are placed on other such items in the electronics industry, such as in the manufacture of flat panel displays, hard disk media, CD glass, and other such workpieces.
Cleaning of a semiconductor workpiece, and other electronic workpieces, occurs at many intermediate stages of the fabrication process. Cleaning of the workpiece is often critical after, for example, photoresist stripping and/or ashing. This is particularly true where the stripping and/or ashing process immediately proceeds a thermal process. Complete removal of the ashed photoresist or the photoresist/stripper is necessary to insure the integrity of subsequent processes.
The actual stripping of photoresist from the workpiece is yet another fabrication process that is important to integrated circuit yield, and the yield of other workpiece types. It is during the stripping process that a substantial majority of the photoresist is removed or otherwise disengaged from the surface of the semiconductor workpiece. If the stripping agent is not completely effective, photoresist may remain bonded to the surface. Such bonded photoresist may be extremely difficult to remove during a subsequent cleaning operation and thereby impact the ability to further process the workpiece.
Various techniques are used for stripping photoresist from the semiconductor workpiece. Mixtures of sulfuric acid and hydrogen peroxide at elevated temperatures are commonly used. However, such mixtures are unsuitable for stripping photoresist from wafers on which metals, such as aluminum or copper, have been deposited. This is due to the fact that such solutions will attack the metals as well as the photoresist. Solvent chemistries are often used after metal layers have been deposited. In either case, limited bath life, expensive chemistries, and high waste disposal costs have made alternative strip chemistries attractive.
Plasma stripping systems provide such an alternative and have been used for stripping both pre- and post-metal photoresist layers. This stripping technique, however, does not provide an ideal solution due to the high molecular temperatures generated at the semiconductor workpiece surface. Additionally, since photoresist is not purely a hydrocarbon (i.e., it generally contains elements other than hydrogen and carbon), residual compounds may be left behind after the plasma strip. Such residual compounds must then the removed in a subsequent wet clean.
As such, one of the more vexing problems in microelectronic device fabrication, such as in the manufacture of semiconductor integrated circuits, is the cleaning of workpieces having exposed metallization at their surfaces. Frequently, such cleaning involves removal of residues formed during the etching and plasma ashing processes associated with forming patterned metallization on the workpiece surface. These residues are commonly called "polymers", but also include inorganic compounds.
To date the most successful methods for removing these residues has involved non-water based wet cleans. These cleans have mainly involved hydroxylamine based reducing chemistries and have been found to be quite effective and robust. The typical polymer removal process also uses IPA as an intermediate rinse agent to eliminate corrosion that could happen in the final DI rinse if the amines in the residue remover mixed directly with water. However, the semiconductor industry is being driven to eliminate or reduce the use of such chemistries because of environmental regulations and cost concerns.
The present inventor has recognized that a water-based replacement for these mixtures would have many advantages over the non-water based chemistries currently employed. This is due to the fact that high pH water based solutions have the ability to dissolve many of the inorganic compounds thought to exist in, for example, post etch residues. Examples of such solutions include solutions of ammonium hydroxide or tetramethylammonium hydroxide. While these solutions etch Ti and TiN films on the order of Angstroms per minute and are thus suitable for use with such films, they etch aluminum films on the order of microns per minute and are thus not suitable for use with workpieces having exposed aluminum metallization. As such, the use of such solutions in residue removal has not been explored in any significant manner.